The invention relates to a method for producing a corrosion-resistant and oxidation-resistant coating on a component wherein the coating is formed from a slurry which is hardened and heat treated after application to the component.
During operation of some components at high temperature, their exposed surfaces are subject to highly corrosive and oxidizing conditions. When used in gas turbines such components may, for example, be made from a superalloy having a nickel or cobalt base. For protection against corrosion, oxidation or erosion, the components are coated with layers made from metal powders.
U.S. Pat. No. 3,741,791 discloses a slurry coating of superalloy substrates, preferably made from Ni or Co base alloys. To improve corrosion and oxidation resistance, a slurry of Si powder and FeCrAlY powder is made, applied to the surface of the substrate, dried and finally heat treated at a temperature of approximately 1200xc2x0 C.
FR 2,115,147 A discloses a coating method for substrates made from Ni or Co superalloys. To improve oxidation resistance, a binder is added to metal powder in the form of a disperse mixture with a particle size of preferably less than 38 xcexcm, the composition is then applied to the substrates and subsequently heat treated. Disclosed metal powders are Coxe2x80x94Al, Nixe2x80x94Al, and/or Fexe2x80x94Al plus Crxe2x80x94Al alloys which can be prealloyed with Y. Optionally, metal powders of Fe, Ni or Co can be admixed.
WO 9407004 A discloses the coating of substrates made of superalloys, in particular Ni base alloys, with a slurry. For improved hot-gas corrosion resistance, the slurry material comprising Al and Si or Cr and Al in a disperse mixture, is applied to the substrate, dried and heat treated at 850xc2x0 C. to 1120xc2x0 C. in an inert atmosphere or in a vacuum.
In a known method for producing a protective coating against corrosion and oxidation, first a metal powder is applied to a component by plasma spraying or the like. Then the coating is exposed to alitizing and finally blazed on. This method is associated with the disadvantages of involving many relatively expensive processing steps, principally the required plasma spraying.
It is thus an object of the invention to provide a method for producing a slurry coating of the type described above in which the method is as simple and cost-effective as possible.
According to the invention, this object is met by the method comprising the steps of forming a slurry by mixing a binder solution with (1) a base powder selected from the group consisting of (a) MCrAlY wherein M is at least one element selected from the group consisting of Ni and Co, (b) NiCrAl, and (c) mixtures of (a) and (b) with (2) an added powder selected from the group consisting of Al, Pt, Pd, Si and mixtures thereof, each of said powders having a grain size between 5 and 120 xcexcm; applying said slurry onto a component made of a superalloy based on nickel or cobalt; hardening the slurry on the component to form a hardened slurry coating on said component by heating the slurry to a temperature between room temperature and 450xc2x0 C.; and heat treating the hardened slurry coating to produce diffusion thereof into the component by heating the coated component at a temperature between 750xc2x0 C. and 1250xc2x0 C. whereby to form a corrosion-resistant and oxidation-resistant coating on said component.
The method according to the invention provides a substantial advantage in that admixing of added powder to the base powder results in an effect similar to that occurring, for example, during conventional alitizing of plasma-sprayed layers, except that the relatively expensive plasma spraying and the alitizing process are eliminated. The method according to the invention can be used both for producing protective coatings on new components and on repaired components.
It has been shown that resistance of the slurry coating to corrosion, oxidation and erosion, is significantly improved by diffusion processes between the base powder and the added powder or the added material.
In an embodiment of the method according to the invention, the base powder comprises MCrAlY and/or NiCrAl, so that coatings with good corrosion protection properties can be produced with such metal powders.
If MCrAlY is used as the base powder, M is Ni or Co or both.
An added powder is mixed with the base powder, the added powder being Al, Pt, Pd, Si or mixtures thereof. The grain size distribution of both the base powder and the added powder ranges from 5 to 120 xcexcm. The powder mixture is mixed with a binding solution to form a slurry.
The slurry is preferably applied to a superalloy component based on Ni or Co by brushing, spraying, dipping and draining or by other suitable methods. In comparison with plasma spraying, these methods result in significant cost advantages.
In an advantageous embodiment of the method, the slurry coating is heat treated for about 2 hours, preferably in a protective atmosphere, for example, in argon or in a vacuum.
Preferably the added powder is present in an amount up to 35% by weight of the total weight of base powder and added powder.